The present invention relates generally to electrophotographic printing systems. In particular, the invention is an image density process control system for a full color electrophotographic proofing system.
Electrophotographic proofing systems are generally known and described, for example, in the Zwadlo et al. U.S. Pat. No. 4,728,983, Cowan et al. U.S. Pat. No. 4,708,459 and Porter et al U.S. Pat. No. 4,780,744. Systems of these types include a computer-based control system, and an organic photoconductor (OPC) which is sequentially driven past charging, exposing (imaging), developing and transfer stations during multiple imaging cycle (toning pass) proofing runs. A separate imaging cycle is performed for each component color used to create the image.
During each imaging cycle the OPC is first charged to an initial voltage by a charging device such as a scorotron at the charge station. The charged OPC is then exposed or imaged to produce a charge pattern representative of the image to be printed. Exposed portions of the OPC are thereby discharged to a final voltage. A bias voltage is applied to the development station to create a development voltage differential between the toning station and OPC. Charged toner is drawn to the imaged OPC as a function of the development voltage and OPC charge profile to develop or tone the imaged OPC as it passes the development station. This imaging cycle procedure is repeated for each component color to produce a composite image assembly in registration on the OPC. The proofing run is completed when the composite image assembly is transferred to a backing by the transfer station.
The amount, and therefore density, of toner applied to the OPC at the developing station is controlled to impart desired color characteristics to the proof. Unfortunately, elements of the electrophotographic process described above have characteristics which change over time and produce unpredictable variations in system dynamics. Two of the most serious process variables are changing charge characteristics of the OPC and changes in the dynamics of the developing system (both toner and mechanism).
The Cowan et al. and Porter et al. patents referenced above describe a half-tone separation proofing system which includes compensation techniques for reducing toner density dependance on process variables. This compensation technique includes the use of four empirically derived mathematical models: a charger model, an exposure model, a decay model and a developer (toning) model. The charger model mathematically predicts the initial or unexposed voltage placed onto the OPC by the scorotron. The exposure model estimates the post-exposure OPC voltages on exposed test areas of the film. The decay model estimates the voltage decay experienced by the OPC as it travels to the developing station. The developer model estimates the density of the toned image given the development voltage. These models are used to predict actual system performance occurring during any toning pass and provide appropriate values of the controlled parameters (grid voltage, bias voltage and exposure setting) to maximize system performance during the next successive toning pass. Actual measurement data is used to update the models at the conclusion of any toning pass. The cycle of performance prediction/parameter estimation followed by model updating is repeated for each successive toning pass.
The control process used in the Cowan et al. system executes two basic phases: calibration and toning. In operation, the calibration phase is run when required. During this phase, the system obtains OPC voltage measurements and estimates certain parameters indicative of the performance of the electrophotographic charging, exposure and decay processes that actually occur in the system. The calibration phase consists of only one pass during which no toning occurs. The result of the calibration phase is a set of parameter values for use during the subsequent toning phase. The calibration phase is run in specific instances before the toning phase begins in order for the system to establish a set of valid initial conditions.
Once the calibration phase, when used, is completed, the toning phase begins. During each successive toning pass, the system first predicts system performance and calculates the values of various controlled process parameters, by inverting the models using updated values from the previous pass or proof, in order to set the controlled process parameters (grid and bias voltages and exposure setting) correctly. Actual process data (toner densities, OPC voltages under conditions of varying exposure and at varying times) occurring during that pass are measured. These measurements are then used to update all the models for use during subsequent toning passes. The performance prediction/parameter estimation and updating processes are again repeated during each successive toning pass.
Electrophotographic systems also generally include systems for replenishing toner consumed during the development process. The Resch, III U.S. Pat. No. 4,847,659 discloses a replenishment control system actuated as a function of a toner depletion signal. The toner depletion signal is indicative of the number of character prints, and is proportionally converted to a replenishment control signal with the proportionality constant being adjusted in response to the difference between a process control parameter such as development bias, and a predetermined target value.
The Ota et al. U.S. Pat. No. 4,886,730 discloses an electrostatic liquid development process in which the replenisher has a different composition of colorant, binder and charge control agent than that of the starting composition. This different composition causes the supplemented developer to hold a state of charge at a predetermined rate.
The Simms et al. U.S. Pat. No. 4,860,924 discloses a liquid developer charge director control for
a copier Liquid carrier is added to maintain the volume of the working developer at a constant level. Toner concentrate is added to maintain optical transmissivity at a predetermined value. Conductivity of the developer is also measured, and charge director added to the working developer to maintain conductivity at a constant value.
There remains, however, a continuing need for improved density process control procedures for electrophotographic systems. The process control procedures must be capable of accurately and efficiently compensating for process variables to repeatably produce proofs having desired color characteristics. No operator interaction should be required to implement the process control procedures. It would also be advantageous if the process control procedures could support a range of operator selected color characteristics.